Abstract: The aerodynamic interference effects on vortex-induced vibration (VIV) of two Π-shaped cylinders in tandem in smooth flow were investigated in the present paper. The section models of the two Π-shaped cylinders in tandem with various separation ratios D/B (D is the net distance between the two Π-shaped cylinders in tandem, B is the width of the single Π-shaped cylinder.) and damping ratios were tested in smooth flow in a wind tunnel. The vortex lock-in intervals and the amplitudes of the windward and the leeward Π-shaped cylinders were compared with those of a single Π-shaped cylinder. The results show that the lock-in intervals of VIV of the windward and the leeward Π-shaped cylinder cylinders are not affected by the factors such as the separation ratio D/B and the damping ratio. However, the amplitudes of VIV of the windward and the leeward Π-shaped cylinders are affected by above factors. The aerodynamic interference effects on VIV of the windward Π-shaped cylinder are mainly affected by the separation ratio D/B, and these effects are the maximum when D/B is 0.5-1.0. The aerodynamic interference effects on VIV of the leeward Π-shaped cylinder are mainly affected by the VIV amplitudes of the windward Π-shaped cylinder. These effects decrease with the increasing of the separation ratio.